1. Field of the Invention
The present invention relates to video recorders, and more particularly to time lapse video tape recorders. Still more particularly, the present invention relates to a time lapse video recorder which includes a monitoring function to ensure that proper signals are being recorded.
In a normal video recorder a continuous stream of video pictures, or fields, is recorded using the helical scan technique. The basic recording configuration is illustrated in FIG. 1. A cylindrical head assembly 10 includes a pair of diametrically opposed video heads 12 and 14. The heads rotate in a direction indicated by an arrow 16, and a moving tape 18 is wrapped around the head assembly in a spiral fashion. The bottom of the tape is thus positioned adjacent to the head path at point A and the top of the tape is positioned adjacent the head path at point B. As a result, video signals are recorded in a diagonal fashion across the tape as illustrated at 20. The direction of tape, travel is indicated by an arrow 22.
The rotating heads are locked to the vertical sync of the video signal such that one diagonal track represents one field. For NTSC systems the heads rotate at 30 rps (1800 rpm). The result is that one complete revolution of the head cylinder will record a complete frame (two fields) of video information, with each adjacent track being recorded by an alternate head, as illustrated in FIG. 2. Thus, tracks 24 will be recorded by head 12 and tracks 26 recorded by head 14. The linear tape speed is selected so that the video tracks are as close together as possible to avoid unrecorded tape without cross-talk between adjacent tracks.
In time lapse recording, the machine does not record every field but rather periodically records fields on tape that is moving at a correspondingly slower speed. For example, in standard NTSC recording, sixty fields per second are recorded. For time lapse video recording having a 6:1 ratio every sixth field would be recorded (ten fields per second) and the tape slowed so that the track spacing remains the same. The coordinated reduction of speed and selective field recording preserves the normal speed format on the tape.
2. Description of the Prior Art
Video recorders are often used in security applications such as the monitoring of a bank or a store. It is therefore important that proper video recordings be made. On the other hand, time lapse recorders are typically intended to be operated unattended for long periods of time. Over these time periods, the video recorder may fail and proper recordings will not be obtained. Failure of the recorder can be caused by many different factors, including oxide buildup on the heads, worn heads, bad spots on the video tape and poor head/tape contact. In addition, tampering with the system, covering or painting the camera lens or cutting the connection between the camera and recorder will prevent proper recordings from being made.